The present invention relates to a latch-up preventive technique for use in a semiconductor integrated circuit with a built-in booster type power supply circuit for generating a voltage resulting from boosting of the source voltage, such as an LSI (large scale semiconductor integrated circuit) for liquid crystal display control with a built-in power supply circuit for liquid crystal display driver, and a technique that can be effectively applied to mobile electric equipment mounted with such an LSI.
Nowadays, as a displaying unit for mobile electric equipment including mobile telephones and pagers, a dot matrix type liquid crystal panel in which a plurality of display pixels are two-dimensionally arrayed in a matrix form for instance is usually employed, and the equipment is mounted with a display control unit, integrated into a semiconductor circuit, for controlling the displaying on this liquid crystal panel of a driver circuit for driving the liquid crystal panel, or a display control unit with such a driver circuit built into it. While such a display control unit integrated into a semiconductor circuit can operated on a voltage of only 5 V or below, displaying on a liquid crystal panel requires a drive voltage of 5 to 40 V. For this reason, the display control unit often has a built-in power supply circuit for liquid crystal display driver for generating a voltage resulting from boosting of the source voltage to drive the liquid crystal panel. More specifically, as shown in FIG. 14, the liquid crystal panel is driven with a segment line drive voltage SEG having an amplitude of about 6 V and a common line drive voltage COM whose amplitude is a few times as great as that (about 40 V).
Power generating systems for a power supply circuit for liquid crystal display driver conceivably include one in which the lower amplitude level VCOML of the common line drive voltage COM is set to the ground potential ground potential (0 V), other levels VCOMH, VSEGH and VSEGL are determined with reference to that and the required voltage is generated by a booster type power supply circuit as shown in FIG. 14A, and another in which the higher amplitude level VCOMH of the common line drive voltage COM is set to the source voltage Vcc (e.g. 5 V), other levels VCOMH, VSEGH and VSEGL are determined with reference to that and the required voltage is generated by a booster type power supply circuit as shown in FIG. 14B.
As the power supply circuit for liquid crystal display driver, a reference voltage circuit or a voltage follower circuit using an operational amplifier is used, and such an operational amplifier operates on the boosted voltage as its source voltage. This, however, involves the problem of considerably high power consumption by the operational amplifier working on the boosted voltage as its source voltage, because the voltage generated by a power supply circuit of the power generation system described is at a considerably high absolute level relative to the ground.